1. Field of the Invention
The present invention is directed to method for operating an x-ray tube of the type having an anode and a cathode arrangement mounted at a distance from the anode and including an electron emitter, the cathode arrangement containing focusing means for the electron beam that emanates from the electron emitter during operation of the x-ray tube, and which is incident on impinges the anode in a focal spot.
2. Description of the Prior Art
Such x-ray tubes of the above general type (see, for example, European Application 0 210 076) are utilized in x-ray imaging systems. In conventional x-ray tubes, an intensity distribution of the x-radiation arises in the focal spot with two humps (peaks). Such an intensity distribution, first, has a negative influence on the modulation transfer function that determines the image quality (with respect thereto, see A. Gebauer et al., "Das Rontgenfemsehen, Georg Thieme-Verlag, Stuttgart, 1974, pages 26 through 33). Added thereto is that the power density, and thus the temperature of the anode is especially high in the region of the two humps in the focal spot. A more favorable curve of the modulation transfer function as well as a maximum temperature of the anode that is theoretically about 10% lower (or a corresponding increase in the power given the same maximum temperature) could be achieved with an intensity distribution similar to a Gaussian curve.
X-ray tubes of this type are also disclosed in European Application 0 115 731, French Patent 26 50 703, U.S. Pat. No. 4,689,809, German OS 43 04 142 and German OS 30 01 141. In these known x-ray tubes a cathode head with a channel that accepts the electron emitter is provided as the electron beam focusing means, the walls thereof that reside opposite one another being chargeable with different potentials in the first three publications in order to enable a dislocation of the focal spot on the incident surface of the anode. By contrast thereto, focal spots of different size are capable of being set in the last two publications. In German OS 30 01 141, this is achieved by a sub-division of the cathode head into a plurality of sections in the longitudinal direction of the channel, these sections being chargeable with different potentials. In German OS 43 04 142, the electron emitter is divided into an uneven number of sections, with either only the middle section being active or pairs of outer sections corresponding to one another being also active.
Focal spots of different size can also be realized in an x-ray tube of the type initially described in German Patent 151 237, by means of two electron emitter sections that are axially displaceable relative to one another and either only the inner, circular section being active, or the outer annular section also additionally active dependent on the displacement position. A focusing of the electron beam is thereby achieved by a concave mirror-like curvature of the electron emitter sections.